The present invention is in a process for conditioning an exhaust gas stream containing dust, before it enters an electrical dust precipitator. The exhaust gas is conditioned by the addition of a conditioning gas containing sulfur trioxide. The SO.sub.3 is produced in a system which includes a furnace with conduits for introducing molten sulfur and combustion air into the furnace, and a gas exhaust conduit for removal of gas from the furnace. The furnace exhaust gas is fed into a catalytic converter connected to the gas exhaust conduit. A conduit carries the gas from the converter to the exhaust gas passage and has a terminal distributing means. The molten sulfur is pumped and metered into the furnace according to a measured magnitude representing the amount of conditioning gas needed. The combustion air is delivered to the furnace by a blower for delivering a sufficient amount of air. An air heater is provided between the blower and furnace. The air heater output is controlled directly in accordance with the amount of sulfur delivered such that the sum of the energy fed to the furnace with the sulfur and with the heated air is constant.
A process for producing a flue gas conditioning gas stream is disclosed in EP-A-O 062 930. It differs from other previously known processes in that the sulfur trioxide producing apparatus is adjusted by an external signal to the sulfur trioxide demand, but that the essential function is taken over internally by a simple control which assures that a constant amount of energy is fed to the furnace for the sulfur combustion, so that the output temperature can be kept substantially constant in a simple manner.
In the known processes, a number of additional possibilities for control and correction are provided, and the result is that, for the person skilled in the art, the central control is either not capable alone of solving the complex control problem, or else requires complex secondary regulating circuits in order to compensate for all fluctuations that occur, such as the ambient temperature, for example.